Active and passive devices employing optical waveguides for use in communication networks are well known in the art and include device structures which incorporate multiple precisely fabricated optical directional couplers on a single integrated optical substrate. Modulators fitting this description are available commercially for distribution of analog signals over optical fiber, noteably for community access television (CATV). These devices are typically formed on a substrate by diffusion of a prescribed material into a substrate. A single substrate may include a plurality of optical waveguide sections and integrated optical directional couplers. An integrated optical directional coupler is formed when two parallel optical waveguides are situated such that their evanescent mode fields overlap. Coupling efficiency is optimized under synchronous coupling conditions which occur when individual single mode waveguide propagation constants are matched. The "interaction length" required for complete coupling of optical power from one optical waveguide to an adjacent optical waveguide is known as the coupler length, L, where L=.sup..pi. /.sub.2K. K is known as the "overlap Integral". The overlap integral and coupler length depend strongly on the device's optical parameters. In fact, the experimental dependence of L (and K) upon waveguide gap separation ("g"), which is determined by fabrication parameters, such as diffusion time ("t") and temperature ("T"), is well known in the art and described in, for example, "Guided Wave Optics", by H. F. Taylor and A. Yariv, Proc. of the IEEE, vol. 62, no. 8, August 1974, incorporated herein by reference as part of the present disclosure.
This dependence unfortunately makes the fabrication of integrated optical directional couplers highly sensitive to the specific values of these parameters. An improperly formed integrated optical directional coupler may reduce the performance of an optical device by, for example, contributing nonlinearities. However, attempting to adjust the characteristics of such waveguides using traditional methods involves altering those same parameters in all waveguide sections on the substrate. The simultaneous diffusion, and thus simultaneous formation of both waveguide sections, makes it difficult to precisely and repeatably form more than one optical directional and couplers on the same substrate. Accordingly, production yields for such a device are often lower than desirable. For example, production yield, A, for a device structure incorporating two optical directional couplers would be proportional to the product of the individual coupler fabrication yields, a, so that A.apprxeq.a.sup.2, where a .ltoreq.1.0.
Waveguides formed by known processes are also susceptible to optical input power having a component with improper polarization. Such an improper polarization can result from, for example, fiber pigtail misalignment, and affects the linear electro-optic affect typically used to induce a modulating refractive index change. Thus, optical input power with an improperly-polarized component may introduce noise into an optical device using the waveguide, and degrade the performance thereof.
As noted, a single process has been used to create the entire optical waveguide device architecture on an individual substrate, including all optical directional couplers. Two such patented processes are U.S. Pat. No. 4,284,663, entitled "Fabrication Of Optical Waveguides By Indiffusion Of Metals" which discloses titanium indiffusion in LiNbO.sub.3 and U.S. Pat. No. 4,984,861 entitled "Low-Loss Proton Exchanged Waveguides For Active Integrated Optic Devices And Method Of Making the Same. The '861 patent discloses an annealed proton exchange (APE) process in LiNbO.sub.3.
Patents which relate to device architectures including multiple directional couplers are U.S. Pat. No. 5,148,503, "Apparatus And Method For Linearized Cascade Coupling Integrated Optical Modulator", and U.S. Pat. No. 5,168,534 for "Cascaded Optic Modulator Arrangement". Examples of relevant journal publications are H. Skeie and R. V. Johnson, "Linearization of electrooptic modulators by a cascade coupling of phase modulating electrodes", Integrated Optical Circuits, vol. SPIE-1543, pp. 153-164, 1991 and W. K. Burns, "Linearized optical modulator with fifth order correction", J. Lightwave Technology, vol. 13, no. 8, August 1995. All of the foregoing patents and publications are incorporated herein by reference.
Accordingly, it is an object of the present invention to provide a method for fabricating a plurality of optical waveguides on a single substrate which does not suffer the above drawbacks.
An object of the present invention is to provide a process which yields an improvement in the fabrication yield of integrated optical modulators, including those devices designed primarily for communications that require multiple optical directional couplers on a single substrate.
Another object of the present invention is to provide a method of the foregoing type which allows for an adjustment in the values of device parameters between first and second optical waveguides on a single substrate.
Still another object of the present invention is to provide a method of fabricating an optical device of the foregoing type utilizing two, non-simultaneous diffusion steps.
Yet another object of the present invention is to provide a method of fabricating an optical device of the foregoing type capable of filtering signals having an undesirable polarization from a received optical beam.
Still another object of the present invention is to provide a method of fabricating an optical device of the foregoing type that interfaces adjacent optical structures formed from disparate diffusion processes, thereby optically "stitching" the structures to one another.